From the viewpoint of heat resistance, it is impossible to use ordinary cables using polyethylene, vinyl, or rubber as the insulating material and the covering material for signal cables and power cables used in places with a high temperature exceeding 300° C., and therefore MI cables are mainly used.
An MI cable is formed by accommodating conducting wires in a metal sheath with an inorganic insulating material powder of magnesia, silica, alumina, or the like disposed between the conducting wires and the metal sheath, and the interior of the MI cable is isolated from the outside air by providing a termination sleeve at its end portion so as to prevent a reduction in insulation resulting from the entry of moisture contained in the outside air into the insulating material powder during transportation or storage, or in use.
A typical conventional structure including an MI cable and a termination sleeve used for places at a temperature of 300° C. or more is shown in the cross-sectional view of FIG. 9, for the case where the MI cable has two conducting wires. The structure is the same for the case where the MI cable has a single conducting wire or three or more conducting wires.
In the interior of an MI cable 1 provided with a termination sleeve 2, two conducting wires 7 run parallel through an inorganic insulating material powder 8 contained in the MI cable 1. The termination sleeve 2 composed of a sleeve tube 4, a ceramic terminal 5, and terminal tubes 6 is provided at a termination of the MI cable 1, and a termination portion of the MI cable 1 is inserted into the sleeve tube 4. The sleeve tube 4 is made of the same material as that of the sheath 3 of the MI cable 1, and the sheath 3 and the sleeve tube 4 are welded around the entire circumference at a weld 14. Also, the other end of the sleeve tube 4 is closed with the ceramic terminal 5, which has two through-holes 5a into which the terminal tubes 6 made of the same kind of metal as the conducting wires 7 are inserted, and the conducting wires 7 extend to the outside through the terminal tubes 6. Each of the terminal tubes 6 and the corresponding conducting wire 7 are welded around the entire circumference at a weld 15. In many cases, the space between the MI cable 1 and the ceramic terminal 5 inside the sleeve tube 4 is filled with an inorganic insulating material powder 9 of magnesia, silica, alumina or the like in order to fix the conducting wires 7 to each other and prevent contact between the conducting wires 7, and between the conducting wires 7 and the sleeve tube 4.
The ceramic terminal 5 and the sleeve tube 4, and the terminal tube 6 and the ceramic terminal 5 are silver soldered around the entire circumference. The interior of the MI cable 1 is isolated from the outside air by the silver soldering, as well as the welding around the entire circumference between the sheath 3 and the sleeve tube 4 and between the terminal tubes 6 and the respective corresponding conducting wires 7, and thus the entry of moisture is prevented.
As for the above-described silver soldering between the ceramic terminal 5 and the sleeve tube 4, and between the ceramic terminal 5 and the terminal tubes 6, the adhesiveness between ceramic and silver solder is poor, and it is therefore common that the bonding surface of ceramic is metallized, then metal-plated, and silver soldered to metal, thus improving the adhesiveness (see, for example, Patent Document 1).